Parasite
http://chipx86.github.com/gtkparasite/

Parasite is a debugging and development tool that runs inside your GTK+ application's process. It can inspect your application, giving you detailed information on your UI, such as the hierarchy, X window IDs, widget properties, and more. You can modify properties on the fly in order to experiment with the look of your UI.

Something that looks like it will make GUI development 30% less painful.